Conventional spring-loaded contact probes generally include an outer receptacle, a movable plunger, a barrel containing the plunger, and a spring for biasing the reciprocating travel of the plunger in the barrel. The plunger is commonly biased outwardly a selected distance by the spring and may be biased or depressed inwardly of the barrel, a selected distance, under force directed against the spring. The plunger generally includes a head or tip for contacting electrical devices under test. Such conventional contact probes are generally referred to herein as three-component probes, since they have a receptacle, barrel and probe member.
In conventional devices, the barrel is mounted within the receptacle with the plunger extending outwardly from the receptacle. Preferably, the barrel is removably mounted in the receptacle, so that should damage occur to the barrel or plunger, replacement is possible. Usually, the receptacle is permanently or semi-permanently mounted in an array within the tester. Electrical wiring may be attached to the receptacle, for electrical communication between the receptacle and the testing diagnostic equipment. Preferably, the probe member, barrel and receptacle are manufactured from electrically conductive materials, so that an electrical circuit is maintained between the electrical device under test and test equipment, by means of the contact probe.
When such electrical probes are used, generally a contact side of the electrical equipment to be tested is brought into pressure contact with the tip of the plunger for maintaining spring pressure against the electrical device. After the electrical device has been tested, the pressure exerted by the spring probes is released and the device is removed from contact with the tip of each probe. In conventional systems, the pressure is released by moving the electrical device and probes away from one another, thereby allowing the plungers to be displaced outwardly away from the barrel under the force of the spring within the barrel, until an enlarged-diameter section of the plunger engages a crimped end portion of the barrel. After repeated test cycles, and repeated instances of the enlarged diameter section contacting the crimped end portion, it has been discovered that the plunger and barrel assemblies become axially displaced from or they "walk out" of their receptacles.
In testing electrical devices using such spring-loaded contact probes, it is desired that the probes each extend a uniform distance from the ends of their corresponding receptacles. This maintains a uniform pressure on the electrical device and ensures that each probe makes reliable electrical contact with the device under test. The plunger barrel assemblies that walk out of their receptacles impose a non-uniform pressure on the electrical device. Since they extend a greater distance from the plane of the test fixture, they can prevent other properly seated plungers from making an electrical contact with the device under test.
Approaches have been taken in the past to solve the problem of plunger barrel assembly walk out. It is desirable that the plunger barrel assembly be releasably fitted within its receptacle to accommodate various switching probe types depending upon the different types of electrical device to be tested. Accordingly, the approaches taken to solve this problem are also aimed at preserving the releasable engagement between the plunger barrel assemblies and their corresponding receptacles, i.e., the plunger barrel assemblies are not permanently affixed to their receptacles.
One approach to reducing plunger barrel assembly walk out has been to form the barrel with a slightly bowed shaped along its axis while the receptacle is unbowed. This produces a releasable friction interference fit which reduces walk-out. However, after repeated test cycling, the interference fit ultimately wears under repeated contact between the plunger and abutting crimped end portion, allowing the plunger barrel assemblies to walk out of their receptacles.
It is, therefore, an objective of this invention to construct a spring-loaded contact probe that eliminates plunger barrel assembly walk out from a receptacle. It is desirable that the spring-loaded contact probe be constructed in a manner that facilitates releasable engagement with the receptacle so as to accommodate use with a variety of different probe designs. The spring-loaded contact probe also should be capable of use with conventional crimp-type receptacles that may or may not already be installed in an electrical test fixture. The resulting contact probe also should accomplish these objectives at a reasonable cost.